The invention relates to the field of instrumentation for optical measurements and more specifically to methods and devices for measuring and correcting for optical aberrations in an optical system.
A typical optical system operates on an incident optical wavefront to transform it to a different optical wavefront. Generally, different points on the wavefront experience different transformations depending on what portions of the optical system they encounter. For example, when a wavefront is incident on a lens, those portions of the wavefront that traverse the periphery of the lens will experience phase delays which differ from those experienced by portions of the wavefront which traverse the center of the lens. A wavefront can be defined as a plurality of points having a constant phase. The wavefront encountering the lens results in a transmitted wavefront having a different shape. Appropriately shaping and positioning lenses can modify a transmitted wavefront to a desired shape.
In some cases, an optical system is known to produce an undesired transformation. One way to correct the transformation is to add a second optical system designed to correct for the deficiencies of the original optical system. For example, in the case of a human eye requiring a corrective lens, the optical components of the human eye perform an optical transformation which is imperfect. In another example, a flawed objective lens installed in a large telescope performs an imperfect transformation. Rather than replacing the objective lens, it may by preferable to install a corrective lens. In both of these cases, it is necessary to know characteristics of the flawed optical transformation in order to correct it.
One method for measuring the optical characteristics of a human eye is the technique of placing lenses having various correction factors in front of the eye and asking the patient whether or not the overall image has improved. Using this substitution technique, one can determine an overall correction for the optical characteristics of the eye. An instrument that is generally used to approximate an optical system that corrects for the flawed optical transformation of an eye is referred to as a xe2x80x9crefractometer.xe2x80x9d In the case of a general lens system, corrections are determined by a variety of tests, each referred to by its owns name, such as the xe2x80x9cFoucault test.xe2x80x9d Throughout the following description, the term xe2x80x9crefractometerxe2x80x9d will be used to refer to all of the instruments that perform such tests.
A mathematical model of the eye can be expressed in terms of a polynomial equation. One such mathematical model is known as the Siedel model. The substitution technique described above determines the overall correction for the eye, but it is limited to prismatic, cylindrical, and spherical corrections. These corrections provide only the lowest-order terms of the Siedel or polynomial model of the eye""s optical system. The technique does not correct for the errors that are specified by higher-order terms of the polynomial model. Additionally, it is not possible to obtain point-by-point measurements of the wavefront at designated sites on the optical system using the technique. For example, where the optical system is a cornea, this technique cannot determine the optimal wavefront portion at each point on the cornea.
A number of refractometers have been developed that are designed to determine the optimal wavefront at designated sites on the optical system. For example, one such optical system includes a reference optical subsystem for projecting a reference pattern on the patient""s retina through a reference area on the cornea and a separate measurement optical subsystem for projecting a measurement pattern on the patient""s retina through a measurement area on the cornea.
To determine the shape of the optimal wavefront at a designated site on the cornea using this refractometer, the measurement pattern is moved across the retina until its location coincides with the location of the reference pattern. Based on the difference between the initial and final positions of the measurement pattern, this refractometer can infer the correction of the wavefront required at the selected corneal site.
An example of another refractometer consists of two optical subsystems aligned along substantially the same optical axis: a reference optical subsystem and a measurement optical subsystem. The reference optical subsystem projects a reference pattern onto a reference pattern position on a detector plane through a selected reference site on the measurement plane. The measurement optical subsystem projects a measurement pattern onto a measurement pattern position on the detector plane through a selected measurement site on the measurement plane. The two subsystems may have some or all of their elements in common.
In operation, the location of the measurement pattern on the detector can be controlled by an observer through the use of an optical aligner coupled to the measurement optical subsystem. Using the optical aligner, the observer can move the measurement pattern on the detector until it is aligned with the reference pattern on the detector. The distance and the direction in which the observer moves the measurement pattern in order to align it with the reference pattern provide a measure of the shape of the optimal wavefront associated with the portion of the wavefront incident on the selected measurement site on the measurement plane. This method is sometimes referred to the xe2x80x9cnullingxe2x80x9d method.
In an alternate operation, a measurement of the displacement of the measurement pattern from the reference pattern is used to characterize the wavefront. This method is sometimes referred to the xe2x80x9cnon-nullingxe2x80x9d method.
Although the devices disclosed above can be used to measure the deviation from the shape of an optimal wavefront at a selected measurement site on the optical system, they are complex and they do not provide an observation of the optical system after correction.
The invention relates to an apparatus for determining a characteristic of an optical element. In one embodiment, the apparatus includes a spatial light pattern generator adapted to generate at least one beam of light at a predetermined spatial position. The apparatus further includes at least one lenslet disposed in an array of lenslets adapted to receive the at least one beam of light from the spatial light pattern generator, and to direct the at least one beam of light to the optical element. The apparatus further includes a detector positioned to receive the beam of light subsequent to the beam of light encountering the optical element, and adapted to detect a received spatial position at which the detector receives the beam of light. The apparatus further includes a processor adapted to compare the predetermined spatial position with the received spatial position to determine the characteristic of the optical element. In another embodiment, the processor is further adapted to change the predetermined spatial position in response to the received spatial position.
In another embodiment, the spatial light pattern generator includes an opaque mask having a movable aperture. In a further embodiment, the spatial light pattern generator includes a spatial light modulator. In yet another embodiment, the spatial light pattern generator includes an array of individually addressable light-modulating elements. In one embodiment, the array of lenslets is arranged in a substantially uniform pattern. In another embodiment, the uniform pattern is chosen from the group comprising substantially a square, a circle, a rectangle, an ellipse, and concentric circles. In yet another embodiment, the detector is chosen from a group of position detectors, including a retina, an array detector, a quadrant detector, a photodetector, a photodiode, a charge coupled device (CCD) detector, and a photosensitive film. In one embodiment, the optical element comprises an eye, a lens, a mirror, a spherical mirror, a segmented mirror, and a flexible mirror. In one embodiment, the processor includes a computer or control electronics. In another embodiment, the apparatus further includes a contact lens fabrication device coupled to the processor. In another embodiment, the apparatus further includes an intraocular lens fabrication device coupled to the processor. In yet another embodiment, the apparatus fiuther includes laser surgical equipment coupled to the processor. In other embodiments, the characteristic includes wavefront aberration, defocus, astigmatism, and curvature.
The invention also relates to a method for determining a characteristic of an optical element. The method includes passing at least one beam of light originating from a predetermined spatial position through a lenslet in an array of lenslets to the optical element. Subsequent to said at least one beam of light encountering said optical element, the method further includes detecting the at least one beam of light at a received spatial position, and comparing the predetermined spatial position with the received spatial position to determine the characteristic of the optical element. The method further includes the step of processing the received spatial position to determine the characteristic of the optical element. The method also includes providing a detector. The detector is chosen from the group comprising a retina, a photodetector, a quadrant detector, a charge coupled device, and a photosensitive film. In another embodiment, the step of passing at least one beam of light includes providing a spatial light pattern generator. In yet another embodiment, the step of comparing the predetermined spatial position with the received spatial position includes providing a processor. In another embodiment, the method further includes the step of changing the predetermined spatial position in response to the received spatial position.
The invention also relates to an apparatus for generating a diffraction limited image. In one embodiment, the apparatus includes a spatial light pattern generator adapted to generate a plurality of beams of light at selected spatial positions to compensate for a characteristic of an optical element. The apparatus further includes an array of lenslets adapted to receive the plurality of beams of light from the spatial light pattern generator and to direct the plurality of beams of light to the optical element. The apparatus also includes an image plane positioned to receive the plurality of beams of light subsequent to the plurality of beams of light encountering the optical element and adapted to form the diffraction limited image. In one embodiment, each of the plurality of beams of light is coherent with respect to the others of the plurality of beams of light.
In another embodiment, the spatial light pattern generator includes an opaque mask having a movable aperture. In a further embodiment, the spatial light pattern generator includes a spatial light modulator. In yet another embodiment, the spatial light pattern generator includes an array of individually addressable light-modulating elements. In one embodiment, the array of lenslets is arranged in a substantially uniform pattern. In another embodiment, the uniform pattern is chosen from the group comprising substantially a square, a circle, a rectangle, a triangle, an ellipse, a pentagon, a hexagon, an octagon and concentric circles. In yet another embodiment, the detector is chosen from the group including a retina, an array detector, a quadrant detector, a photodetector, a photodiode, a charge coupled device (CCD) detector, and a photosensitive film. In one embodiment, the optical element comprises an eye, a lens, a mirror, a spherical mirror, a segmented mirror, and a flexible mirror. In one embodiment, the processor includes a computer or control electronics. In another embodiment, the apparatus further includes a contact lens fabrication device coupled to the processor. In another embodiment, the apparatus further includes an intraocular lens fabrication device coupled to the processor. In yet another embodiment, the apparatus further includes laser surgical equipment coupled to the processor. In other embodiments, the characteristic includes wavefront aberration, defocus, astigmatism, and curvature.
The invention also relates to a method for generating a diffraction limited image. The method includes passing a plurality of beams of light through a lenslet array to an optical element, the plurality of beams of light originating from selected spatial positions to compensate for a characteristic of the optical element. Subsequent to said plurality of beams of light encountering the optical element, the method further includes imaging the plurality of beams of light to form a diffraction limited image. In one embodiment, each of the plurality of beams of light is coherent with respect to the others of the plurality of beams of light.
In another embodiment, the step of imaging includes providing a detector. The detector is chosen from the group comprising a retina, a photodetector, a quadrant detector, a charge coupled device, and a photosensitive film. In another embodiment, the step of passing a plurality of beams of light comprises providing a spatial light pattern generator. In yet another embodiment, the step of imaging comprises providing a processor.